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Old 01-26-2019, 07:45 AM   #1
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Maximum Recommended PV Array Voltage

I am getting quotes from different PV installation folks for a PV array I want to put on my RV roof. The last installer has quoted me an 8 panel system of 170W per panel for a total of 1360W. He has recommended wiring all of the panels in series which would result in 182V/9.32A (Voc/Isc) and 155V/8.79A (Vmpp/Vimpp). The Victron Energy 250/100 charge controller can support these high voltages just fine and deliver approximately 95A @ 14.6V to the battery bank (12V/840AH AGMs). I have 6AWG prewired to the roof.

My question is about safety. At 182V, we're not fooling around here. Everything will be fused and there will be breakers before and after the charge controller, but I'm wondering about the wisdom of this design.

What do people think? I don't know the electrical code recommendations here... and Googling hasn't helped. Input appreciated!
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Old 01-26-2019, 08:08 AM   #2
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It sounds like that guy does house installs and doesn't understand possible shading issues on an rv. Why not series/ parallel or parallel? What's his reasoning for such high voltage? Plenty of us out here with similar amount solar and not pushing high voltage and living as if we're plugged in to the grid but out in the bush.
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Old 01-26-2019, 09:07 AM   #3
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It sounds like that guy does house installs and doesn't understand possible shading issues on an rv. Why not series/ parallel or parallel? What's his reasoning for such high voltage? Plenty of us out here with similar amount solar and not pushing high voltage and living as if we're plugged in to the grid but out in the bush.
The installer is an RV installer... I think the argument is that if you have 8 panels, and 1 panel is shaded, isn't it just as likely as 2 panels shaded and unluckily shading panels on the separated series strings?

My rig has awnings up on the roof that are about 8 inches high. I'm going to have a racking system installed so the panels are completely in the clear of anything on the roof (e.g. A/Cs, antennas) that could shade any panel. So it is only external shading..which is entirely random and changes throughout the day.

Decisions .. decisions
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Old 01-26-2019, 09:35 AM   #4
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Originally Posted by SteveUpp View Post
I am getting quotes from different PV installation folks for a PV array I want to put on my RV roof. The last installer has quoted me an 8 panel system of 170W per panel for a total of 1360W. He has recommended wiring all of the panels in series which would result in 182V/9.32A (Voc/Isc) and 155V/8.79A (Vmpp/Vimpp). The Victron Energy 250/100 charge controller can support these high voltages just fine and deliver approximately 95A @ 14.6V to the battery bank (12V/840AH AGMs). I have 6AWG prewired to the roof.

My question is about safety. At 182V, we're not fooling around here. Everything will be fused and there will be breakers before and after the charge controller, but I'm wondering about the wisdom of this design.

What do people think? I don't know the electrical code recommendations here... and Googling hasn't helped. Input appreciated!
Since it's over 60VDC as long as they run the cables in conduit from the roof to the charger it's OK.

However I would suggest split the panels into two 4 panel strings which would keep the voltage down to under 100VDC then use two smaller Victron MPPT chargers. That way if you ever have a charger failure you only loose half of your panels.

Bottom line is that either will work just fine and don't worry about shading issues, see my comments below, as it seems your installer understand shading.

Quote:
Originally Posted by Itinerant1 View Post
It sounds like that guy does house installs and doesn't understand possible shading issues on an rv. Why not series/ parallel or parallel? What's his reasoning for such high voltage? Plenty of us out here with similar amount solar and not pushing high voltage and living as if we're plugged in to the grid but out in the bush.
I'm so tired of people repeating the falsehood that one should parallel series strings or run all panels parallel because it's totally the wrong way to do it. What they want to do it TOTALLY right.

Too many people, there are some here that do get it, here think that if a panel gets shaded in a series string you will loose the entire string. This is not correct, you only loose the one panel and in some cases on part of the panel.

The reason is that each group of cells, there are usually 2 or 3 groups in each panel, has a bypass diode which allows the other cells to keep producing power.

So if one panel gets shaded in a series string the other panels still produce power because the power flows through the bypass diode.

However, if you parallel two series strings and one panel gets shaded then you will loose that entire string because its total voltage drops below the other series string which has full sun. This is why it's best for each series string to have it's own MPPT controller.

The installers for home solar understand this. They typically have 20 or more panels in series but if some of the panels gets shaded the others still produce power. They also know that you should never parallel series strings so they will run an inverter per series string.

So flat out the best way to install panels is in series with each series string feeding it's own MPPT charge controller. You then parallel the outputs of the MPPT charge controllers.

FYI, Being an engineer and I understand this, it's basic electrical theory, so I challenge anyone to prove me wrong.
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Old 01-26-2019, 10:19 AM   #5
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Since it's over 60VDC as long as they run the cables in conduit from the roof to the charger it's OK.

However I would suggest split the panels into two 4 panel strings which would keep the voltage down to under 100VDC then use two smaller Victron MPPT chargers. That way if you ever have a charger failure you only loose half of your panels.

Bottom line is that either will work just fine and don't worry about shading issues, see my comments below, as it seems your installer understand shading.


I'm so tired of people repeating the falsehood that one should parallel series strings or run all panels parallel because it's totally the wrong way to do it. What they want to do it TOTALLY right.

Too many people, there are some here that do get it, here think that if a panel gets shaded in a series string you will loose the entire string. This is not correct, you only loose the one panel and in some cases on part of the panel.

The reason is that each group of cells, there are usually 2 or 3 groups in each panel, has a bypass diode which allows the other cells to keep producing power.

So if one panel gets shaded in a series string the other panels still produce power because the power flows through the bypass diode.

However, if you parallel two series strings and one panel gets shaded then you will loose that entire string because its total voltage drops below the other series string which has full sun. This is why it's best for each series string to have it's own MPPT controller.

The installers for home solar understand this. They typically have 20 or more panels in series but if some of the panels gets shaded the others still produce power. They also know that you should never parallel series strings so they will run an inverter per series string.

So flat out the best way to install panels is in series with each series string feeding it's own MPPT charge controller. You then parallel the outputs of the MPPT charge controllers.

FYI, Being an engineer and I understand this, it's basic electrical theory, so I challenge anyone to prove me wrong.
Exactly right, many years ago, panels did not have bypass diodes and you could lose a complete panel , and if in series, the whole string if any of them were partially shaded. I am running two strings of 3 panels ea in series to two mppt controllers. If one panel in either string is partially shaded, my voltage and current may fall a bit, but the non shaded areas and panels will still produce full power.
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Old 01-26-2019, 03:17 PM   #6
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OP, you may want to look into my “800W solar install” posted here as I have the raised rails for the awnings and came up with a simple way of tilting them.
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Old 01-28-2019, 06:04 AM   #7
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Series is the way to go. The higher voltage means you can use smaller wires which will be cheaper and easier to run. 180 volts really isn't anything unusual. I mean a lot of the world uses 240 ac for their domestic wiring and the insulation on the wire is probably rated for 600 volt. As long as the charge controller is good for it just treat it the way you would any of your 120 volt wiring and don't worry about it.
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Old 02-18-2019, 12:36 PM   #8
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I am not sure if it matters or not, but a very common DC breaker used in solar installs are the midnight solar units.

These breakers are actually made by a different company, and relabeled by a number of sellers. At $20, the are a bargain.

IIRC, they are rated for 150 volt DC, which I think is based on the assumption of Voc at lowest expected temperature.

If you go down a technical path that will only work with one brand of components, it can be potentially troublesome in the future.

IMHO, it is worth the effort to wire the system in a way that it can stay within the 150 VDC limit.

Perhaps I am being too conservative.
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Old 02-18-2019, 08:21 PM   #9
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Old 02-18-2019, 08:33 PM   #10
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If system components are rated at XX then it's best not to exceed XX. There are times you can oversubscribe if its known the usage senario will not exceed.
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Old 02-18-2019, 08:39 PM   #11
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Some home grid tie systems run to 600 volts in series. I run 120 volts on a 150 volt solar controller with no problems.
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Old 02-18-2019, 10:40 PM   #12
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I’d just stick with inline fuses located at the panel. Remember the fuse it to protect the wire so you want it at the source. Less expensive and more reliable. Use one per series string. If your rightunning more than one string don’t parallel the strings but use a MPPT controller for each series string.

https://www.amazon.com/Renogy-30-Wat.../dp/B00YG2IAQ4
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Old 02-18-2019, 11:54 PM   #13
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Id just stick with inline fuses located at the panel. Remember the fuse it to protect the wire so you want it at the source. Less expensive and more reliable. Use one per series string. If your rightunning more than one string dont parallel the strings but use a MPPT controller for each series string.

https://www.amazon.com/Renogy-30-Wat.../dp/B00YG2IAQ4
I have never really been impressed with the renogy components, but it is still amazing to me that a company can sell a fuse holder and still have multiple complaints about quality. Look at the amazon reviews. Unbelievable.
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Old 02-19-2019, 08:57 AM   #14
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I have never really been impressed with the renogy components, but it is still amazing to me that a company can sell a fuse holder and still have multiple complaints about quality. Look at the amazon reviews. Unbelievable.
I'm not a Renogy fan but 86% positive is a pretty good rating. After reading the other 14% I would chalk up most to purchaser issues.
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